Question 11

The following table gives information on the proportions of a population that have been exposed to a risk factor for a disease and then subsequently developed the disease.

Exposure +      Indicates the proportion exposed to the risk factor (A+B)

Exposure -       Indicates the proportion not exposed to the risk factor (C+D)

Disease +         Indicates the proportion that subsequently developed the disease (A+C)

Disease -         Indicates the proportion that did not subsequently develop the disease (B+D)

Disease +

Disease -

Exposure +

A

B

Exposure -

C

D

Define prevalence AND, with reference to A, B, C, D in the table above, give the prevalence of the disease in this population.           (20% marks)

Define relative risk (RR) AND, with reference to A, B, C, D in the table above, derive the relative risk Of developing the disease after exposure to the risk factor. (40% marks)

Define attributable risk (AR) AND, with reference to A, B, C, D in the table above, give the attributable risk of exposure to the risk factor on developing the disease in this population. (40% marks)

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College answer

a) Prevalence: number of event (e.g. disease) in a specific population at a particular time point.

Prevalence of the Disease in this population:

A+C / (A+B+C+D)

b) Relative risk is the ratio of the probability of an event occurring (e.g. developing a disease) in an exposed group to the probability of the event occurring in a comparison, in non-exposed group

[A / (A+B) ] / [ C / (C+D)]

c) Attributable risk is the difference in the rate of a condition between an exposed and unexposed population.

A/ (A+B)-C/(C+D)

Discussion

This is another SAQ which makes it very easy to earn high marks, as it asks for unambiguous memorised definitions and has a clear-cut right answer.

Somebody got 9.2.

Prevalence:

  • The proportion of individuals in a population having a disease or characteristic in a particular population at a given time.
  • Prevalence = number of affected individuals / total number in population
    = (A+C) / (A+B+C+D)

Relative risk:

This is the difference in event rates between 2 groups expressed as proportion of the event rate in the untreated group. The slightly broken English of the college answer probably comes from an article similar to the 2017 article by Tenny et al, and was probably meant to say "relative risk is a ratio of the probability of an event occurring in the exposed group versus the probability of the event occurring in the non-exposed group."

  • RR =  absolute risk in treatment group / absolute risk in control group
    (absolute risk = number of cases in group / total number of group)
  • Thus, RR = [A/(A+B)]/[C/(C+D)]

Attributable risk:

  • This is a measure of the absolute effect of the risk of those exposed compared to unexposed. It indicates the number of cases of a disease among exposed individuals that can be attributed to that exposure
  • AR = Incidence(exposed) – Incidence(unexposed) 
    (incidence = number of cases / population at risk)
  • Thus, AR = ( A / A+B) - (C /C+D)

References

References